A typical environment where static charge build-up in fluidized beds is a serious problem 6 in thermal testing of electronic components.
Generally, thermal shock testing involves subjecting an article to be tested, i.e., the workpiece to sudden heating and cooling by moving it to and from separate hot and cold environments. Thermal cycling tests involve changing the temperature of the thermal environment without moving the workpiece. These and related methods are hereinafter referred to collectively as Thermal Testing unless the context indicates otherwise.
Thermal Testing is extremely useful for evaluating the reliability of materials and devices because the thermal stresses created in such workpieces by sudden temperature changes tend to accelerate failures due to defects that would otherwise be undetectable until long after manufacture. For example, thermal testing is widely used in the electronics industry to detect defective semiconductor devices before they are installed in complex electronic systems where failure could result in expensive repairs and considerable down time.
A typical prior art device for conducting thermal testing would have an enclosed heat transfer medium provided with a heating means, e.g., electric resistance heaters, and/or a cooling means, e.g., gas expansion refrigeration. Specifically, a thermal cycling device would have a single heat transfer medium enclosure with both heating and cooling means, and a thermal shock device would have two such enclosures, one for cooling and the other for heating. In operation, the workpiece is simply submerged in the heat transfer media for a fixed period of time, and its temperature changes until it reaches or approaches thermal equilibrium with the heat transfer media.
The nature of the heat transfer media is a dominant factor in how fast the workpiece is heated or cooled, i.e., the rate at which the system approaches thermal equilibrium. Gaseous heat transfer media, usually air, have the advantage of being inexpensive and not contaminating the workpiece, thus avoiding subsequent cleaning operations before use. However, the heat transfer characteristics of gases are slow and hence they cannot change a workpiece's temperature fast enough to create sufficient thermal stresses to satisfactorily accelerate failures due to latent defects in the workpiece.
Liquids unlike gases, have very fast heat transfer characteristics and high heat capacities. Therefore, they make excellent heat transfer media for thermal testing. A workpiece submerged in liquid heat transfer media rapidly reaches or approaches thermal equilibrium with the system and sufficient thermal stresses to induce failure of the workpiece due to latent defects are attained. However, liquids also have drawbacks that make them undesirable for many thermal testing applications. Specifically, since suitable liquids that are stable at the temperatures contemplated for thermal testing, usually -65.degree. to 200.degree. C., are usually very expensive and evaporation, dripping, and carryout losses are usually large, rendering the costs of such testing very high.
Volatility and toxicity of such liquids may also pose serious health hazards, fire hazards, and problems with governmental regulatory agencies as well. An additional problem with liquid heat transfer media in thermal testing devices is that the liquid may have to be cleaned from the workpiece to avoid problems in subsequent use, e.g., soldering.
Moreover, since few, if any liquids would be suitable for both extremely high and extremely low temperature applications some thermal shock systems using different high and low temperature liquid heat transfer media, i.e., liquid/liquid systems, are subject to cross-contamination caused by carryover when the workpiece is moved from one liquid to the other.
The disadvantages of liquid and gas thermal testing systems may be overcome by the use of fluidized solids is the place of gaseous or liquid heat transfer media.
One property of gas fluidized solids as a heat transfer medium that causes a problem in some thermal shock applications is that it tends to build up static charges due to the flow of fluidizing gas around the particles. In applications involving sophisticated electronic devices such as programmed chips, the static charges may affect the devices adversely.